Resistance of Rhipicephalus microplus to amitraz and cypermethrin in tropical cattle farms in Veracruz, Mexico.

The objectives of this study were: (1) to determine the prevalence and factors associated with Rhipicephalus microplus , resistant to cypermethrin and amitraz, from cattle farms in Veracruz, Mexico, and (2) to determine in vitro mortality percentages of field populations of R. microplus exposed to discriminating doses (DD) of cypermethrin and amitraz. Fifty-three populations of R. microplus were tested by bioassays using DD of cypermethrin (0.05%) and amitraz (0.0002%). The prevalence of cattle farms with R. microplus ticks that were resistant to cypermethrin and amitraz, and co-resistant to both acaricides, was 90.6, 54.7, and 47.2%, respectively. The level of cypermethrin resistance, measured as a survival percentage, was higher as compared to amitraz. Cattle farms with ≤50 animals (odds ratio [OR]  =  3.84, 95% confidence interval [CI]  =  1.07-13.70, P  =  0.038) and a stocking density of >1 animal unit per ha (AU/ha) (OR  =  4.21, 95% CI  =  1.0-17.71, P  =  0.050) had a higher probability to develop R. microplus tick populations co-resistant to both acaricides. In conclusion, there is a high prevalence of R. microplus populations on cattle farms in Veracruz, Mexico that are both resistant to cypermethrin and amitraz and co-resistant to both acaricides. The level of cypermethrin resistance is critical, and the exposition variables of ≤50 cattle and a stocking density of >1 AU/ha were factors associated with R. microplus co-resistant to both acaricides.